In electron beam welding the effect of gravity on the molten mass in the welding zone with increased thickness of the welded workpieces more and more outweighs the surface tension of the molten mass, which tends to hold back the molten mass in the welding zone. When welding with a vertically extending electron beam it is therefore in practice already necessary with workpieces of at least 50 mm thickness to place supporting strips etc. underneath in order to prevent the molten material from flowing out of the welding zone.
The problem of preventing the molten mass from flowing out of the welding zone is not quite so grave when welding thicker workpieces by means of a horizontally extending electron beam, since in this case not the whole weight of the molten mass acts on the free surface of the molten material on the front and rear side of the workpiece. With increasing workpiece thickness, however, generally from about 100 mm upwards, even when welding with a horizontally extending beam, it is impossible to prevent the molten mass from flowing out of the welding area. Attempts have been made to overcome this unwelcome feature by placing "backing" strips underneath the actual welding zone at the front and rear sides of the workpiece which are intended to stop the molten mass from flowing out of the welding zone. However, for geometric reasons this measure cannot always be used. On the contrary, it very often produces additional costs, sometimes very substantial ones, and very often even these measures cannot prevent the molten mass from flowing out of the welding zone.